Si3N4 and SiAlON based ceramic materials have been intensively investigated during the last decades due to their superior mechanical properties with good thermal stability and excellent thermo-shock properties. These properties have a wide range of applications and used such as ceramic cutting tools, ceramic bearings, ceramic substrate, space industry, and continues to receive attention in the automotive component market. As compared with carbide-based materials, or steel materials, silicon nitride generally offers the potential of relatively high heat resistance and chemical stability, relatively low density, good mechanical properties such as hardness and toughness, and good electrical insulation characteristics. To illustrate the advantages, in the context of the cutting tool industry, these properties can combine in whole or in part to allow operations to proceed at higher speeds and temperatures, with resulting potential cost savings. The potential market for the above properties indicates use in other applications, such as, extrusion dies and automotive components, turbocharger components, swirl chambers, and engine valve.
Single-phase of Si3N4 is a high covalent compound and exist in 2 hexagonal polymorphic crystalline forms α- and β-Si3N4, β-Si3N4 being stable than the α form. The structure of α- and β-Si3N4 is build up from basic SiN4 tetrahedra joined in three-dimensional network by sharing corners, with common nitrogen to the three tetrahedral sites. Either structure can be generated from the other by a 180° rotation of 2 basal planes. The α- to β-Si3N4 transition is usually by a solution-precipitation reaction of Si3N4 and molten glass. The strong covalent bonds of Si3N4 produce some mechanical and engineering properties for these materials such as: low thermal expansion coefficient, which results to good thermal shock resistance, high strength which results to high toughness, greater Young's modulus than some metals, thermal stability, up to 1800° C., which is temperature when Si3N4 starts to decompose. The weak point of this material is difficulties of self-diffusion and production of Si3N4 into a dense body by classical method of ceramic processing technology. This problem can be helped to a large extent by using sintering additives, glass-formers and formation of sialons by substituting silicon and nitrogen with aluminium and oxygen.
Nitrogen rich sialon phases have been extensively studied in connection with the development of high performance ceramics, especially in α- and β-sialon systems [T. Ekström and M. Nygren, J. Am. Ceram. Soc., 75, 259 (1992)]. The structure of α-Si3N4 was established using single crystal X-ray diffraction (XRD) data and film methods [R. Marchand et al, Acta Cryst. B25, 2157 (1969)] and more accurate atomic positions were obtained in later single crystal XRD studies [I. Kohatsu et al, Mat. Res. Bul., 9, 917 (1974) and K. Kato et al., J. Am. Ceram. Soc., 58, 90 (1975)]. Structural changes of α-Si3N4 with temperature, below 900 C have also been investigated using neutron powder diffraction data [M. Billy et al., Mat. Res. Bul., 18, 921 (1983)]. The α-Si3N4 crystallises in the space group P31c with the unit cell parameters a=7.7523(2), c=5.6198(2) Å, V=292.5 Å3 [Powder Diffraction File 41-0360, International Centre for Diffraction Data, Newtown Square, Pa.] and unit cell content Si12N16.
The α-sialons are solid solutions that have a filled α-Si3N4 type structure. There are two substitution mechanisms. First, silicon and nitrogen can be substituted simultaneously by aluminium and oxygen. Second, the structure has two large, closed cavities per unit cell that can accommodate additional cations of metals, M=Li, Mg, Ca, Y, and Rare Earth (RE) elements. A general formula for α-sialons can thus be written as MxSi12−(m+n)Al(m+n)OnN16−n, where x (=m/v)≦2, and v is the average valency of the M cation. For all of the known α-sialon compositions the m/(m+n) ratio is found to be below 0.67. Examples of reported α-sialon phases are Y.5 (Si9.75 Al2.25) (N15.25 O0.75) and Ca.67 (Si10 Al2) (N15.3 O0.7) [F. Izumi et al., Journal of Materials Science, 19, 3115 (1984)]. This is likely due to the synthesis approach that is usually used with metal oxides or carbonates of M=Li, Mg, Ca, Y, and RE as additives used either as substitution in α-sialon crystal structure or as glass-formers and sintering additives. The addition of M in the form of oxides incorporates oxygen atoms in the sialon system.
In the Swedish patent application SE 0300056-9 is described a method for obtaining nitrogen rich glass phases by using non oxide additives. The mechanical properties of the nitrogen rich glass phases have been reported to be improved with the increased nitrogen content.
Even though α-sialon phases have been used in many different commercial applications, specially as single phase ceramics or together with other compounds in composite ceramics, and despite an intensive scientific investigations and developments in this field there has been crucial limitations in the chemical compositions of the crystalline α-sialon phases as well as in the intergranular glassy phase found in the ceramic bodies produced.
One of the important uses of sialon-based ceramics or silicon nitride or silicon carbide is their high temperature applications. The most important parameter for high temperature applications is the inter-granular glass phase in the ceramics. By increasing the nitrogen content and thereby getting better mechanical properties and higher glass transition temperatures the obtained ceramics show much better high temperature stability with respect to chemical stability as well as mechanical stability.